Abdominal role in squats

Question

Many people, websites,videos and wikipedia imply that squats, specially weighted squats train the abdominal muscles. But squatting without contracting my belly makes it that the abdominal pressure pushes the belly out making it easier for my lower back and enabling me to use heavier weight.

Contracting my abdominals sucks inward my belly making the squat almost impossible.
I can do front squats with belly relaxed using 70 kilograms for 20 repetitions.

By activelly contracting my abdominals I can't even do one repetition with the same weight.

So, how does the squat even train the abdominals?

Answer 1

Squats train all of the supporting muscles of the torso -- including the anterior/abdominal muscles -- if you use a Valsalva maneuver.

Consider performing each squat repetition using these steps:

1. Inhale as deeply as you can.
2. Hold your breath by closing your glottis ("throat"), not just your lips.
3. Contract your torso muscles hard, including your abdominal muscles. Do not either "suck inward" or relax outward.
4. Squat downward, and return to the starting (standing) position.
5. Release the held breath only after completing the movement.

The Valsalva maneuver in the squat is basically identical to holding a deep breath and preparing to be punched in the abdomen or to push a stalled car.

How does the squat train the abdominal muscles? As you squat progressively heavier weights, abdominal contraction plays an increasing role in maintaining spinal extension (somewhat counter-intuitively): The abdominal muscles help keep the torso rigid by compressing the air trapped in the lungs, but only if you don't allow your abdomen to bulge outward. This resistance to lengthening the abdominal muscles under load provides the stress via which squats train those muscles.

Answer 2

It is a bit more complicated than that.

The main abdominal muscles are the rectus abdominis, external and internal obliques, and the transverse abdominis. The purpose of these muscles is two-fold: on the one hand, they move the spine (bending forward and twisting), on the other hand, they are the muscles used for forced exhalation.

These two functions are pronounced differently, depending on how deep the muscle is. The main function of the more superficial muscles (the rectus abdominis and the external obliques) is bending and twisting, and the main function of the deepest muscles (mainly the transverse abdominis) is compression and forced exhalation.

Given this complexity, it is not surprising, that when people say they are "contracting the abs", they can mean different things. Some people mean as if they are trying to do a crunch (bending the spine), some mean exhalation. Of course, most people always do a bit of both, as it is not easy to isolate the abdominal muscles (though it is definitely possible to isolate them, and it looks super weird). But people can be using different muscles slightly differently, according to their natural disposition.

When lifting heavy objects, like squatting with weight, it is natural to hold the breath. As described in Christian's answer, the Valsalva maneuver is forced exhalation without letting the air out. It increases abdominal pressure, like an inflated tire, it makes the torso more rigid and stable. Many people do this naturally without being told. The main muscle responsible for increasing this intra-abdominal pressure will be the transverse abdominis:

... the co-ordinative patterns shown between the muscles of the ventrolateral abdominal wall are task specific based upon demands of movement, torque and stabilization. It appears that transversus abdominis is the abdominal muscle whose activity is most consistently related to changes in intra-abdominal pressure. [1]

It is normal, that, when the intra-abdominal pressure increases, the abdomen is pushed out a little bit. The stomach being pushed out a little bit does not mean, that the transverse abdominis is inactive, it is still working to balance out the pressure, which helps a lot with squatting.

I suspect, that when you say you are contracting your belly consciously, you are actually contracting the more superficial muscles, like when doing a crunch. They do not help much with squatting, in fact, they can make it harder, because they work to bend the spine forward, so you are working against yourself in the squat. Also, when you say you are doing front squats with "belly relaxed", you are probably contracting the deeper muscles, you are just less conscious of it.